1. Field of the Invention
The present invention relates to a nitride-based compound semiconductor substrate and a method for fabricating the same.
2. Description of the Related Art
A nitride-based compound semiconductor such as GaN has been remarked as a material for a ultraviolet light emitting diode and a blue light emitting diode, since the nitride-based compound semiconductor is one of direct transition semiconductors and has a large bandgap.
As a substrate for fabricating a nitride-based compound semiconductor light emitting diode, a foreign substrate such as sapphire substrate has been used. However, there are disadvantages in that a high-density dislocation occurs in accordance with a heteroepitaxial growth and that a manufacturing process of the light emitting diode is complicated.
For solving the above problems, a nitride-based compound semiconductor self-standing substrate has been actively developed and researched. As a representative example of the nitride-based compound semiconductor self-standing substrate, Japanese Patent Laid-Open No. 2003-178984 (JP-A-2003-178984) discloses a method for fabricating a GaN self-standing substrate by forming a thick GaN layer on a foreign substrate such as sapphire substrate by using a HVPE method (Hydride Vapor Phase Epitaxy), and removing the foreign substrate after formation of the GaN layer. In this method, a layer having a void functions as a strain reducing layer (strain relaxation layer), which relaxes a strain due to a difference in lattice constants or difference in thermal expansion coefficients between a backing substrate and a group III nitride-based compound semiconductor. According to this structure, it is assumed that the group III nitride-based compound semiconductor substrate with a low defect density and an excellent crystallinity without warping can be obtained. According to this technique, a GaN substrate with a low dislocation is now realizing and begins to appear in the market.
However, when fabricating an ultraviolet light emitting diode from the nitride-based compound semiconductor, there are disadvantages in that the GaN substrate absorbs the ultraviolet light and that a brightness of the light emitting diode falls. So as to prevent these problems, it is effective to provide a substrate made of a ternary mixed crystal that is expressed as AlxGa1−xN (D<x<1), since the bandgap can be increased by using Al.
However, an AlGaN crystal having a sufficient area and thickness as a substrate for epitaxial growth does not exist till now. The main reason is that an aluminum chloride generated in the growth reacts so intensely with a quartz reacting furnace.